Transverse deuteron spin relaxation studies of a smectic liquid crystal polymer: Local motions, order director fluctuations, and the glass transition process

摘要

Molecular dynamics of a main chain thermotropic liquid crystal polymer in the smecticAphase has been investigated using multipulse dynamic nuclear magnetic resonance (NMR) techniques. Transverse deuteron spin relaxation timesT2ECPfrom quadrupole echo pulse trains (modified Carrndash;Purcellndash;Meiboomndash;Gill sequence) measured for deuterons in the aromatic rings of the mesogenic units are obtained as a function of pulse spacing tgr; sample orientation thgr;N, and temperature. Just below the nematicndash;smecticAphase transition, the relaxation times exhibit a linear dispersion regimeT2ECPsim; tgr;minus;1consistent with smectic director fluctuations. At lower temperatures, the dispersion step gradually disappears, indicating that faster molecular motions are the dominant transverse relaxation process. The observed anisotropy inT2ECP, measured at short pulse spacings, approximately follows the (sin4thgr;N)minus;1dependence expected for axial diffusion in a highly ordered medium. Analysis of the experiments is achieved employing a density operator treatment based on the stochastic Liouville equation. The intramolecular motion is identified with phenyl ring flips and is the fastest process studied, with correlation times varying from 10minus;10to 10minus;7s over the temperature range investigated.Intermolecular (individual molecule) dynamics are somewhat slower and have been interpreted as rotational diffusion in an orienting potential. The correlation times for intermolecular motion exhibit nonhyphen;Arrhenius behavior approaching the glass transition, following a temperature dependence described by the Williamsndash;Landelndash;Ferry equation over six orders of magnitude. This result indicates a strong coupling of the intermolecular motion to the glass transition process. The slowest motion affecting transverse deuteron spin relaxation is assigned to smectic director fluctuations or undulation waves. Analysis of theT2ECPdispersion yields information concerning the viscoelastic properties of the polymer. AtT=418 K, a splay elastic constant ofK1=2times;10minus;11Nhas been estimated. Using the experimentally accessible value for the long wavelength cutoff of the elastic modes, the root mean square fluctuation lang;thgr;20rang;1/2of the director is calculated to be 4deg;.